An extinction event (also known as: mass extinction; extinction-level event, ELE) is a sharp decrease in the number of species in a relatively short period of time. Mass extinctions affect most major taxonomic groups present at the time ? birds, mammals, reptiles, amphibians, fish, invertebrates and other simpler life forms. They may be caused by one or both of: · extinction of an unusually large number of species in a short period. · a sharp drop in the rate of speciation. Over 99% of species that ever lived are extinct, but extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine invertebrates and vertebrates every million years.Marine fossils are mostly used to measure extinction rates because they are more plentiful and cover a longer time span than fossils

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